There's nothing especially fun or challenging about it. You stand around and DPS dragons. It lasts way too long, and the dragons have such low health that most of my DoT spells really don't do much of anything. I can only imagine how bad it must be for melee players, who barely have time to even get to the dragons before they're dead. And as a final insult, the damn dragons don't even drop loot.
Trash pulls are never really much fun in general, but Ultraxion trash is definitely a Dragon Soul low point. That's my opinion, but how about you -- what trash pull do you think is the worst? How you define worst is up to you: most challenging, most annoying, most time-consuming, hardest to tank, hardest to heal -- whatever makes you want to defenestrate your keyboard the most.